The user asking how to build a master Lego inventory with bi-directional syncing is asking the wrong question. They are treating a symptom, fragmented worksheets, when the real problem is that their data is not designed to be a single, living source of truth. Combining three sheets into one master sheet is straightforward. Getting them to update each other automatically in both directions is not only possible but should be the default expectation for anyone managing a collection that changes over time.
Here is what that means in practice. If you copy and paste all rows from your Marvel, Star Wars, and minifigure sheets into one combined sheet, you have a static snapshot. The moment you edit a set number on the master, the original Marvel sheet will stay outdated unless you manually repeat the merge. The solution is not a manual combine; it is a relational structure. In a true data platform, one that understands relationships between records, you define each Lego set once, in one place. The Marvel sheet, the Star Wars sheet, and the minifigure sheet are then simply filtered views of that single master list. Update the master, and every view reflects it instantly. Update a view, and the master updates too, because they share the same underlying data.
This is exactly what AI-native spreadsheet tools are built to do. They treat your data as a database, not a grid of disconnected cells. The user's current approach, three separate worksheets with overlapping information, is the same pattern that causes version-control chaos in business spreadsheets. The fix is to stop thinking in sheets and start thinking in records. A Lego set is a record. Its theme, piece count, and condition are fields on that record. A minifigure is a separate record that can be linked to a set. When you structure data this way, combining and syncing become automatic. No formulas, no manual reconciliation, no fear that an edit in one tab will break the others.
The concrete takeaway is this: do not build a master sheet by merging three tabs. Build one sheet that contains every set and minifigure as unique records, then use filters or dynamic views to separate by theme. Any tool that cannot give you bidirectional syncing between views is not saving you time, it is creating future work. The user's question reveals they already understand they need a single source of truth. The next step is to adopt a tool that treats that truth as a living, connected system rather than a static copy.